1. Field of the Invention
This invention relates to medical devices which are implanted in the human eye, particularly to a valve which is easy to manufacture, performs reliably, is easy to surgically implant in the eye, and will remain functional for the life of the patient in which it is implanted. The valve of this invention has a unique shape and flexibility that improves performance and reduces the possibility of irritation when implanted on the eye of a glaucoma patient.
2. Background Discussion
Medical valves are used in a wide variety of applications within the human body. One such application is to treat glaucoma in patients where pharmacological treatment is not desirable. In this application, a valve is used to allow aqueous humor to flow in a controlled manner from the intraocular chamber of the eye to relieve excess pressure.
Several devices have been developed for relieving intraocular pressure. Thomas C. White in U.S. Pat. No. 4,554,918 has suggested one type of glaucoma valve where the aqueous humor flows from the intraocular chamber through a tube into an external reservoir. The end of the tube in communication with the reservoir has a very small opening that restricts the flow of aqueous humor into the reservoir. The reservoir, once filled, must be pressed periodically by the patient to force the contents through another tube into the body of the patient, where it is absorbed.
Another device used to treat glaucoma is discussed by Anthony C. B. Molteno in U.S. Pat. No. 4,457,757. This device includes a rigid plate having a tube in communication with the intraocular chamber. The aqueous humor flows from the intraocular chamber onto the surface of the plate and is absorbed by the body. The Molteno device does not have any pressure controlling mechanism, relying solely on the absorption rate of the bleb formed around the plate to maintain proper intraocular pressure.
A third device, suggested by George Baerveldt et al. in U.S. Pat. No. 5,476,445, is similar to the Molteno device except that Baerveldt uses a flexible plate which is often tucked under the muscles. The larger surface area is assumed to help reduce intraocular pressure over longer periods. But like Molteno, Baerveldt does not address the problem of hypotony. Since Baerveldt does not have a valve system, the doctors have to rely on other mechanisms to control the immediate outflow of aqueous humor causing hypotony.
These devices have been used to treat surgically glaucoma patients, but they do have disadvantages. The White valve suffers from the disadvantage that the patient must manually press the reservoir in order to force aqueous humor collected in the reservoir to escape and be absorbed by the body. Moreover, although the White valve is designed to maintain a proper amount of intraocular pressure, the valve's structure is not reliable because it depends upon a tiny opening in the end of the tube, which can easily become clogged with particulates. Nor is the White valve sensitive to minute pressure changes, since it merely limits flow rather than opening and closing at predetermined pressures.
The Molteno plate overcomes the objections of a manually actuated reservoir. However, it does not employ a valve to control pressure loss in the intraocular chamber, which creates the risk of hypotony. The Baerveldt device improves upon the Molteno plate, but does not mitigate the possibility of hypotony because it too does not utilize a pressure controlling mechanism to maintain intraocular pressure at an appropriate level. The flexibility of the Baerveldt device eases installation, but also creates the possibility that the flexible plate may collapse during installation, significantly reducing the absorption area available.
A valve invented by the Applicant and disclosed in U.S. Pat. No. 5,071,408 (herein referred to as the Glaucoma Valve) has many features that distinguish it from White, Molteno, and Baerveldt. This Glaucoma Valve uses a membrane under tension to form a trapezoidal chamber having an elongated, slit-like opening therein. The membrane responds to the slightest changes in pressure to expand or contract to open or close the opening. When opened, it provides a wide open mouth with parted lips that allows for free flow of fluid through it without any substantial resistance to fluid flow. This feature also substantially reduces the likelihood that the opening will be clogged by particulates. The trapezoidal configuration of the chamber renders the valve highly responsive to slight changes in pressure. In this application, the valve maintains a pressure of 10.0 millimeters (mm) of mercury (Hg), with an increase of 0.5 mm of Hg opening the valve. As soon as intraocular pressure reaches 10.0 mm of Hg, the valve is totally shut off to prevent further flow of aqueous humor from draining from the intraocular chamber. Thus, the Glaucoma valve utilized in this invention solves both the problem of over pressurization of the intraocular chamber due to blockage, and the problem of excessive loss of aqueous humor leading to hypotony. Two plates hold between them in tension the overlying membrane members which form between them the chamber. The slit-like opening is along adjoining, overlapping edges of the membrane members. Preferably, the membrane members are simply two halves of a thin sheet of elastic material having an hour-glass shape which is folded over upon itself. The two plates each include interlocking members that, upon the plates being pressed together, engage to place the membrane members disposed between the plates in tension.